In recent years, various techniques for calculating a correct current time and date are disclosed based on GPS signals from GPS satellites for use in a navigation system.
GPS system currently in use worldwide adopts a time series that started at a reference time of 00:00:00 (a.m.) on Sunday, Jan. 6th, 1980. In this time series, the length of 1 second is the one defined in Coordinated Universal Time (UTC). The GPS signals from the GPS satellites include an elapsed time from the reference time, that is, so-called GPS time data for time measurement.
The GPS time data is made up from WN data that represents the number of weeks from the reference time and the number of elapsed seconds from 0 a.m. Sunday of the week specified by the WN data. The WN data is represented by using 10 bits of data, that is, a total of 1024 weeks from 0th week to 1023rd week. Then, the WN data “rolls over” to return to 0th week for measurement.
Therefore, the GPS time data in the above-described format is only capable of representing the time series from 0 a.m. of the 0th week of Sunday, Jan. 6th, 1980 to 11 p.m. 59 min. 59 sec. of the 1023rd week of Saturday, Aug. 21, 1999, and no further date and time.
However, the time series in the GPS time data is as accurate as the time series in UTC except for the portion of the Christian Era (the portion of years), thereby providing an accurate time measurement when the Christian Era year is corrected for the use in the navigation system.
For example, Japanese patent document JP-A-2002-90441 discloses a technique that the Christian Era year in the GPS time data is corrected based on a correct time information derived from a reference radio wave.
However, the technique in the above document uses a dedicated communication device for receiving the reference radio wave, thereby demanding an increased room in the navigation system. Further, the technique in the above document does not work properly when the reference radio wave is not available.